Restricted hydraulically damped mounts are those which have a mechanism to prevent excessive travel of one end portion relative to the other, particularly in the tension mode. Such bumper stops have been disclosed in U.S. Pat. Nos. 4,607,828 and 4,650,168. Non-restricted mounts are those that lack that particular feature and rely on the strength of the rubber spring as well as its adhesion to the related metallic portions to prevent their coming apart. The present invention can be used in both types of mounts.
The dynamic rate is a measure of the spring stiffness of a mount at a given load and a given amplitude at varying vibration frequencies. It has been found that at higher frequencies, for example, 100 to 300 Hz, non-restricted mounts generally increase in dynamic rate. This increase in spring stiffness translates into noise and vibrations which may be transmitted between the power unit being suspended and the supporting frame. It was discovered that for restricted mounts having a bumper stop in the form of a cylinder with an outwardly extending flange, the dynamic rate in the same frequency range had a tendency to be decreased. However, it was understood that the diameter of the bumper stop had to be restricted in order to prevent rubbing or other contact between the flange and the side of the mount which can occur during any radial relative movement of one end of the mount with respect to the other.
In view of those considerations, it is an object of the present invention to provide a hydraulically damped anti-vibration mount in which the dynamic rate at higher frequencies is considerably reduced. It is a further object of the invention to provide a mount which avoids the detrimental effects of contact with the side of the mount by a rigid portion of the structure contained therein.